feat: Add Google VertexAI Provider Support

[Saturn225]

No description provided.

[Saturn225]

Need minor tweaks in models.rs . I will wrap up and push it too

[CLAassistant]

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[nirga]

@Saturn225 is this ready for review?

[Saturn225]

@nirga Yes, it's ready for review. Just need minor tweaks in models.rs and tests to be added

[nirga]

@Saturn225 please follow the issue guidelines and send for review when you've done everything - including tests etc